Deposition of a synthetic polycrystalline diamond thin film on a substrate surface is generally accomplished at low pressures by thermal or plasma breakdown of a hydrocarbon in the presence of hydrogen. Morphology of the synthetic diamond film is governed by the nucleation and growth of individual crystalline grains on the substrate. Although various techniques for growing diamond films are well known, improved methods of growing polycrystalline diamond films are being investigated by numerous commercial and government organizations. Many investigators are pursuing high pressure arc or flame techniques for rapid bulk growth, while some are attempting to increase the area coverage with microwave plasma sources. This great interest in the field of diamond formation has developed because synthetic polycrystalline diamond film has potential as a high temperature semiconductor and offers a combination of highly desirable physical properties, such as hardness, chemical inertness, transparency to light from ultraviolet to far infrared, and resistance to laser damage.
The primary methods used to form polycrystalline diamond include: 1) DC plasma jet, formed by a simple DC discharge into a flowing gas; 2) microwave plasma chamber, using a low pressure microwave cavity at 10 to 100 torr of gas, to form an internal plasma ball; 3) RF discharge between parallel plates; 4) RF discharge using a coil and a flowing gas tube; 5) RF discharge using inductive coupling to produce a one atmosphere plasma torch (ICP); 6) hot filament reactor using a 2000.degree. C. filament to break up hydrogen and hydrocarbon thermally at low pressures; 7) microwave torch using a 2.45 GHz microwave magnetron with a copper rod to absorb microwave energy and produce a plasma jet; 8) oxy-acetylene or other hydrocarbon burning torch having a temperature in excess of 2000.degree. C. in the main flame; and 9) microwave torch using a lossy cavity to induce a low pressure plasma in a closed-flow tube.
Each of the foregoing techniques has some particular limitation when applied to the fabrication of diamond films on a large scale. These limitations include incorporation of metal impurities from anodes and cathodes that are used to produce arcs, incomplete combustion and formation of graphite in the flames, an inability to rotate samples uniformly in tuned microwave cavities, and a generally slow rate of diamond growth. Filament growth systems eliminate some of these problems, but they have other limitations such as filament breakage and highly localized deposition. Thus, there is a need for improved techniques to form high quality polycrystalline diamond films efficiently and at high rates of growth.